Coen Development of a High Performance Versatile Low ... - Inproheat

The mixture of primary gas and air is very lean. This mixture gradually burners in the furnace asit mixes with combustion products of the core, hot furnace gases and secondary fuel. Thiscombustion process is mostly taking place at the lean limits of flammability, thus formation of bothprompt and thermal NOx are greatly reduced.The secondary fuel ignition point is also delayed as the furnace gases in the area around theburner has very low oxygen content. Gradual burning starts as the secondary fuel engagesoxygen from the combustion air both prior and after the combustion of primary gas.The core fuel is a small percentage of the overall fuel flow. It provides a reliable ignition sourcefor the primary and secondary fuel.An overall interleaving pattern of air and fuel distribution creates conditions for a sufficientlycompact overall flame and low carbon monoxide emissions with low excess air.As the mixing of primary gas and air takes place inside the burner, flash back can happen at lowloads. This problem was addressed during the early development stage by simply allowing flashback to occur. This typically happens with one or two slots of air and primary gas flow at theloads below 20%. At these reduced loads flash back is continuous, but does not cause anydamage to the burner. Each small flash back flame is surrounded by considerable amount ofcombustion air and is not in contact with the burner parts. The overall turn down of the burner is1.2 to 1.3 times of the turn down in the combustion air flow. Typically turn down of 8:1 to 10:1can be met with the basic QLN design without a significant increase in CO emissions. Forspecial applications requiring wider turn downs a special modified QLN burner was developedthat is capable of a turn down up to 2.5 times that of the turn down of the combustion air. Withthe adequate fuel controls this easily translates into the burner turn downs over 15:1. This higherturn down modification, however, shows slightly higher NOx.The above description is based on the observations of the flame patterns generated by burnersof different sizes operating under different conditions. CFD modeling was only moderatelyused in the burner design, as accurate predictions of ignition points flame fronts, etc. are stillvery difficult and not reliable for this complex three dimensional problem even with modern CFDcodes like Fluent. Some useful results came from the mixing model between primary gas andair that helped in the optimization process. Most of the work, however, was done experimentally.The burner also shows good NOx performance when firing oil. This is attributed to the effect ofenhanced recirculation of combustion products in the part of the furnace adjacent to the burnerand the effect of air staging. As the major portion of combustion air discharges into the furnacein a spoke star shaped pattern the entrainment of rate of furnace gas by combustion air is greatlyenhanced, if compared with the entrainment by a single round jet. This creates reduction in theflame temperature and reduced thermal NOx production. At the same time, diverting a certainportion of air flow to the ports around the burner throat creates air staged combustion, when mostof the fuel burns in reduced oxygen environment, thus reducing NOx formation from fuel boundnitrogen as well as thermal NOx.BURNER PERFORMANCE AND OPERATING PARAMETERSThe first prototype of the burner was built with the throat diameter of 20”. The second prototypehad 26” diameter throat and was rated to 60 MMBtu/Hr. Test firings were performed in an eightfeet diameter water cooled furnace partially lined with refractory. The furnace had numerousobservation ports and ports for the flue gas sampling. The very first prototypes of the burner hadlimited stability range with respect to variations in the excess air. After the improvements to thedesign, however, combustion stability was reliably maintained up to 9-10% O2.The test data shown in Fig. 2 were plotted versus excess air as it appeared to be the mainparameter determining the NOx for any given burner modification within the wide range of loads.The effect of firing rate on NOx in the test furnace was relatively small. The deviation of data isprimarily due to different burner adjustments.The size of the visual flame body and CO emissions measured with a traversing probe atdifferent distances from the furnace front were also a strong function of the overall excess air. At3